A multi-level digital print device can lay down different amounts of colorant at each addressable location on the media using multiple levels for one or more of the colorants. This ability is common for digital inkjet printing devices and electrophotographic (EP, toner) printers.
Several different approaches have been used for the physical application of the colorant to the media. For inkjet devices, for instance, any of the following may be used: (1) The print head moves across the media multiple times, emitting ink drops on every pass. Each location on the media may receive zero, one, or more drops. (2) The media moves beneath a ‘grayscale’ print head, where each nozzle is capable of delivering a variety of different drop sizes on demand. (3) The media moves beneath a series of print heads, each nozzle of which can emit a single drop size, but each addressable location on the media may have a drop placed by zero, one, or more heads. (4) The media moves below a series of print heads, which contain nozzles, each of which can deliver a single drop size, but where different nozzles can emit different drop sizes. This list is not intended to be exhaustive.
The amounts of colorant that can be applied in any location on the media form discrete steps. Throughout this document the term “colorant level” is used to describe one of these steps. A RIP (raster image processor) or screening engine that delivers rasters for imaging by the printer marking engine may or may not wrap the screened raster in different ways, depending on how the multiple levels of colorant are applied, but the same screening principles apply in all cases.
A screening technology designed for multi-level screening typically defines a response curve for each colorant level. The response curve, for each input tone value, determines what percentage of the addressable locations on the media should be covered with that colorant level.
The PostScript Language Reference Manual discusses promotion of binary (1-bit) screens to multi-level screens by dividing the full tonal range into a number of sub-ranges equal to the number of colorant levels. The first colorant level may be filled, for example, from 0 to 100% coverage through a first sub-range of tones (for example, 0 to 0.7), and then the second colorant level may replace that first colorant level through a second sub-range of tones (for example, 0.7 to 0.9), and so on.
This approach has several challenges or difficulties in practice. First, there are often artifacts at the cross-over points between colorant levels. For example, the transition from one colorant level to the next can exhibit less texture than at other tone values. This can be particularly visible across long, smooth tonal graduations. Second, the approach assumes that the incremental tonal contribution from each colorant level will be more or less equal. In practice, this assumption is usually not justified. Multi-level screens are typically non-linear. The first level often accounts for most of the output tonal range. Third, the approach constrains the screen design to use the same pattern of coverage within the screen for every colorant level when, in contrast, it may be valuable to use different patterns of coverage.
The sub-range over which each colorant level is used is therefore normally adjusted to more or less in line with the density achievable with that colorant level, and those ranges may also be overlapped to avoid texture artefacts at the cross-over points. In some cases, colorant levels do not make use of their full potential coverage because that causes additional challenges, e.g. ink failing to dry on an inkjet printer or toner flaking from an EP printer.
There is a need for methodologies to determine a tone response curve for each colorant level that matches or approximates an aim curve (i.e., a target curve).